In impact-type apparatus for generating seismic signals, a weight, usually in the neighborhood of 5,000 pounds, is propelled at high speed at a base plate which is located in engagement with the surface of the ground. Naturally, the initiation of propulsion of weight of this magnitude and its impact with the base plate creates large dynamic forces, with reaction forces occurring as a result thereof.
In apparatus of this type, the base plate is generally shock-isolated from a frame which supports the movable weight and the means for propelling the movable weight. Accordingly, it is possible for relative movement to occur between the base plate and the frame supporting the movable weight as a result of either the impact between the weighted member and the base plate or the reaction forces.
During the early development of impact-type apparatus, the weight was propelled vertically into a generally horizontally disposed base plate. Although there could be some relative movement, the base plate tended to remain in a relatively fixed position with respect to the frame supporting the weighted member. Recently, however, it has been determined that it is desirable to set the axial centerline of the weighted mass and the propulsion means at an angle relative to the surface of the base plate. When used in this manner, vector forces along the ground surface are created, which tend to cause or at least aggravate the problem of maintaining the base plate in alignment with the propelled mass.
Such apparatus are referred to as full waveform seismic sources and one such apparatus is illustrated in U.S. Pat. No. 4,660,674 issued on Apr. 28, 1987 to Tom P. Airhart. U.S. Pat. No. 4,721,181 issued on Jan. 26, 1988 to Tom P. Airhart illustrates one attempt to compensate for such movement.
In addition, it is frequently desirable to generate the seismic signals on uneven terrain. If, for example, a vehicle carrying a seismic signal generator is setting on a hillside, the weight of the vehicle will not be directly over the base plate and, thus, aggravates the problem created by tilting the actuator.
The problem has become further aggravated because it is now desirable to utilize a multiplicity of such devices, usually on different vehicles, to create the seismic shock. To provide the highest quality seismic signal, each of the masses must strike the base plate at precisely the same time.
In an effort to prevent excessive recoil motion of the vehicle, the weight of the vehicle is shock isolated against the base plate. Even though such apparatus weighs many thousands of pounds, a reaction force still occurs that can cause movement between the supporting structure or frame and the base plate. If such movement occurs, the mass will not strike the base plate in the same position along the same path each time such that it will not strike at an accurately predictable and repeatable time from initiation of propulsion to impact.
The importance of having propelled masses of multiple units impact the plates at precisely the same time can be appreciated by those skilled in the art of the interpretation of the seismic traces. Specifically, it is important that the seismic traces, which are recorded by geophones positioned at specified distances from the source of the seismic signal, be as highly defined as possible. In the event that the masses do not strike the base plates at the same time, more than one seismic signal is generated in very close sequence, producing a rather fuzzy or blurred seismic trace as recorded by the instruments. Such blurred traces are more difficult to interpret and are more difficult to use in the computer systems processing such signals. It is sometimes desirable or necessary to utilize signals of opposite polarity for the purpose of cancelling noise and other undesirable signals that may occur or may be picked up by the geophones. If more than one signal is generated, it may be difficult or impossible to conveniently handle the signals in processing.